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Constraints on the seismogenic faults of the 2003–2004 Delingha earthquakes by InSAR and modeling
Journal of Asian Earth Sciences 75 (2013) 19–25
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Constraints on the seismogenic faults of the 2003–2004 Delingha earthquakes by InSAR and modeling Xianjie Zha ⇑, Zhiyang Dai Laboratory of Seismology and Physics of Earth’s Interior, University of Science and Technology of China, Hefei 230026, Anhui Province, China
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Article history: Received 22 January 2013 Received in revised form 19 June 2013 Accepted 27 June 2013 Available online 9 July 2013 Keywords: Interferometric SAR Co-seismic slip Delingha earthquakes Numerical simulation
a b s t r a c t The 2003 Mw6.3 Delingha earthquake happened at the Dacaidan-Zongwulong fault system, which is an important active tectonic belt on the northeastern margin of Tibetan plateau. Until the end of 2004, 6 approximately Mw5.0 aftershocks occurred on almost the same seismogenic fault. The conventional viewpoint is that the seismic hazard near the continental seismogenic fault will be reduced in decades or centuries after rupture. Why did so many large earthquakes happen on almost the same seismogenic fault within a short temporal interval? The seismogenic fault information and the character of the co-seismic slip distribution are the keys to answer this problem. In this paper, the co-seismic slip distribution of the 2003 Mw6.3 Delingha earthquake was inverted using 18 teleseismic broadband P waveforms, 8 SH waveforms and 28 long period surface waveforms data. The peak slip concentrates at 12 km depth beneath the epicenter, and the slip decreases gradually from 6 km depth to surface and beneath 12 km depth. The shallow slip deficit of the co-seismic slip distribution may be caused by inelastic failures in the uppermost crust. Beneath 12 km depth, the strain energy may be accumulated by the velocity-strengthening frition behavior and may trigger aftershock. Three post-seismic InSAR interferograms spanning the most aftershocks were formed. Through analysis on the aftershock distribution and the focal mechanisms of Mw P 4.9 aftershocks, two 3D finite element models containing topography were built and used to simulate the InSAR observations. Comparing the modeled results with the InSAR observations, we obtained the seismic source of the 2003–2004 Delingha earthqakes. It consists of two southwest-dipping seismogenic faults, named the west-fault and the east-fault. The east-fault extends beneath the west-fault. The 2003 Mw6.3 Delingha earthquake occurred on the west-fault, while 6 large aftershocks occurred on the east-fault. This shows that the complex spatial location relationship between the seismogenic faults may be an important factor to the large earthquake occurrence with high frequency at almost the same locality. Due to special tectonic setting and complex structure, the Dacaidan-Zongwulong fault system will be a high seismic risk zone in the future. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction On 17 April 2003, an Mw6.3 earthquake hit Delingha region, Qinghai province, China. Until the end of 2004, more than 180 Mw P 2.0 aftershocks and 6 Mw P 4.9 aftershocks occurred in this region (see Table 1 and Fig. 1). According to the focal mechanisms given by GCMT and Sun et al. (2012), these earthquakes occurred on the west part of the Dacaidan-Zongwulong fault system, which is an important active tectonic belt between the Qaidam basin and the Qilian Mountain on the northeastern margin of the Tibetan Plateau (see Fig. 2). Moreover, the epicenters of Mw P 4.9 aftershocks are nearly located on the same seismogenic fault. Similarly, Elliott et al. (2011) studied the 2008 and the 2009 Mw6.3 Qaidam ⇑ Corresponding author. Tel.: +86 0551 63607203. E-mail address: [email protected] (X. Zha). 1367-9120/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2013.06.013
earthquakes and found that those earthquakes occurred on almost the same seismogenic fault in the different depths. Generally speaking, the seismic hazard of the seismogenic fault in the continental crust will be reduced in serval decades or centuries after the rupture releasing the strain energy. Why can so many earthquakes with similar magnitude happen on almost the same seismogenic fault within a short time interval?. The seismogenic fault geometry and the character of the coseismic slip distribution are the keys to answer the above problem. In addition, the seismogenic fault information is the base to study seismic source, seismic risk, regional geodynamics, and so on. Due to the sharp topography in the Delingha region, it is very difficult to study the seismogenic faults by the field geological investigation. According to the previous results, Jiang et al. (2006) inferred that the dip of the seismogenic faults is toward southwest (see Fig. 3 in Jiang’s paper). However, Sun et al. (2012) studied a part
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Table 1 The focal mechanism solutions of the 2003 Mw6.3 Delingha earthquake and its aftershocks. The data is derived from GCMT.
a b
Time
Longt. (°)
Lati. (°)
Dep. (km)
Magn. (Mw)
Fault Strike
Plane Dip
2003/4/17 2004/2/24 2004/3/02 2004/3/16 2004/5/04 2004/5/04 2004/5/10
96.45 96.61 96.52 96.56 96.65 96.62 96.58
37.53 37.66 37.61 37.60 37.60 37.61 37.55
16.0 23.0 21.1 13.0 18.6 21.6 12.0
6.3 5.0 4.9 5.1 5.3 5.2 5.5
116 141 100 127 145 140 116
61 87 41 42 83 87 59
Solutiona Rake 91 179 93 116 175 178 103
Typeb Thrust-slip Strike-slip Thrust-slip Thrust & strike-slip Strike-slip Strike-slip Thrust & strike-slip
The fault plane solution is inferred from the GCMT focal mechanisms and the distribution of aftershocks with Mw P 2.0. The fault type is derived from the focal mechanism.
of aftershocks of the 2003 Mw6.3 Delingha earthquake and inferred that the seismogenic faults are dipping to northeast according to the aftershock distribution. Up to now, the seismogenic fault geometry of the 2003–2004 Delingha earthquakes is still unkown. In addition, Elliott et al. (2011) proposed that depth segmentation of the seismogenic continental crust allows a significant seismic hazard to remain after large earthquake. To look for the cause of the frequent occurrence of large earthquakes on almost the same seismogenic fault, this paper used the InSAR observations, the teleseismic waveform data and numerical simulation to constrain the seismogenic fault and analyzed the seismic risk of the Dacaidan-Zongwulong fault system.
Fig. 1. The 2003–2004 Delingha earthquake series map. The data is derived from the Network of Earthquake Information of China. The local magnitude of the main shock is 6.7, corresponding to moment magnitude 6.3. Until the end of 2004, there are more than 180 aftershocks (6 aftershocks with moment magnitude P4.9) occurred in the source region.
Fig. 2. The tectonic setting map. The Dacaidan-Zongwulong fault is an important active tectonic belt between the Qaidam basin and the Qilan mountain, on the northeastern margin of Tibetan Plateau. Open stars denoting the 2003–2004 earthquakes occurred on the west part of the Dacaidan-Zongwulong fault system. The topography of this region is very sharp.
2. Co-seismic slip inversion Before the 2003 Mw6.3 Delingha earthquake, no SAR data covering the source region is available. Because the moment magnitude is up to 6.3, the main shock could be recorded by the global distributed seismic stations, whose epicenter distances are from 30° to 90°. The co-seismic slip distribution can be inverted using teleseismic broadband waveform data. To invert the co-seismic slip of the 2003 Mw6.3 Delingha earthquake, we collected the focal mechanisms from GCMT and the GSN broadband waveforms data from the IRIS seismic network. Based on data quality and azimuthally distribution, we analysed 18 teleseismic broadband P waveforms, 8 SH waveforms and 28 long period surface waveforms data. Two possible fault planes are separately chosen to invert the slip distribution. One fault plane’s strike, dip and rake are 116°, 61° and 91° separately. The other’s are 294°, 29° and 88° separately. The epicenter is located at (96.45°E, 37.53°N). The 1D crustal model is from the Crust2.0 (Bassin et al., 2000). We utilized the finite fault inversion algorithm to invert the co-seismic slip distribution of the 2003 Mw6.3 Delingha earthquake. For the details of the inversion technique, please refer to the paper of Ji et al. (2002). Through comparing the waveform fits, we found the first fault plane (strike = 116°, dip = 61°, rake = 91°) fitting the data is better. In Fig. 3, (a) is the distribution of the seismic stations, (b) is the source time function of the 2003 Mw6.3 Delingha earthquake, (c) and (d) are the observed and the modeled body waveforms and surface waveforms, respectively. The co-seismic slip distribution is shown in (e). In figure (e), one can see that the slip distribution extends from 2 to 20 km in depth direction. The peak slip is up to 40 cm, which is concentrated at 12 km depth beneath the epicenter. The slip distribution is asymmetrical, and the slip magnitude on the east side of the fault plane is larger than that of the west side. From 6 km depth to surface and below 12 km depth, the slip decreases gradually. The rupture length on the Earth’s surface is about 20 km, but that in the depth of 12 km is more than 30 km.
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Fig. 3. Co-seismic slip distribution and observed (black) and modeled (red) teleseismic waveforms. (a) Is the station distribution. The star denotes the seismic source, the triangles denote the seismic station, (b) is the source time function of the event, (c and d) are separately indicating the body waveform and surface waveform fits, (e) is the co-seismic slip distribution of the 2003 Mw6.3 Delingha earthquake. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Post-seismic InSAR observations After the main shock, the Advanced SAR sensor of the Envisat satellite acquired many SAR data sets covering the source region
of the 2003 Mw6.3 Delingha earthquake. We collected 7 scenes SAR data sets from European Space Agency. In addition, we collected the meteorological data of the Delingha region. The meteorological conditions of the Delingha during 2003–2004 are very
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favorable to map the deforation by InSAR. In terms of less vegetation in this region, the deformation interferogram derived from the collected SAR data sets should be with good coherence. However, we processed more than 8 InSAR image pairs with small perpendicular and temporal baseline, and found that the most pairs could not produce the ideal interferometric fringes. After the events, Sun et al. (2003) reported that landslides occurred in the epicenter region. This is a possible explanation to the low coherence of SAR image pairs. At the southeast corner of the coverage of the SAR image, there is a flat area. The correponding interferometric fringes in three InSAR maps is very clear (see Fig. 4). Those are the only observations of the post-seismic process in the near field. Three interferograms span the period from 28 November 2003 to 25 June 2004. Following the 2003 Mw6.3 Delingha earthquake, a series of aftershocks happened in the source region. Two aftershocks with moment magnitude of 5.0 and 4.9 occurred in the period of Fig. 4(a). An Mw5.1 aftershock occurred in the period of Fig. 4(b). Three aftershocks with moment magnitude of 5.3, 5.2 and 5.5 occurred in the period of Fig. 4(c). Therefore, the deformations induced by these aftershocks should be recorded by three post-seismic interferograms. 4. The seismogenic fault The seismogenic fault geometry is an important basic information to study the process of seismic source (Wang et al., 2012; Shan et al., 2011). According to the aftershock location and the focal mechanisms of the Mw P 4.9 earthquakes from Sun et al. (2012) and GCMT, we inferred that there may exist two seismogenic faults, whose strikes are about 116° and 140° separately (see Table 1 and Fig. 5). In this paper, the seismogenic faults are named the west-fault and the east-fault. According to the focal mechanism of the 2003 Mw6.3 Delingha earthquake, the west-fault is the seismogenic fault of the main shock. We assumed that its length along strike is 40 km and its dip angle is 60°. From the coseismic slip distribution, we inferred that the depth of west-fault is about 20 km. However, the parameters of the east-fault are uncertain. We assurmed that its length along strike is about 56 km and its dip angle is 45°. Because the depths of some aftershocks near the east-fault are larger than 20 km, the east-fault may cut through the upper crust and its depth approximates to 22 km. The finite element method is widely used to model the deformation field induced by large earthquakes (Hu et al., 2004; Wang
Fig. 5. Aftershock locations and focal mechanisms of the 2003–2004 Delingha earthquakes. The open cycles denote the aftershock distribution in the Earth’s surface, and the crosses denote the aftershock distribution in the depth direction. The focal mechanism solutions are determined by GCMT. Two red lines denote the strike of the seismogenic fault inferred from focal mechanisms and aftershock distribution. The dashed box denotes the coverage of the SAR data used in this paper. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
et al., 2007; Abolghasem and Grafarend, 2003). To further constrain the dips of the seismogenic faults of the 2003–2004 Delingha earthquakes, we built two 3D finite element models containing topography with different dips to model the InSAR obervations. In Fig. 6, (a) and (b) are the view of the 3D finite element models. The seismogenic faults’ dips in (a) and (b) are separately toward southwest and northeast. According to the previous crustal
Fig. 4. The post-seismic interferograms of the 2003 Mw6.3 Delingha earthquake. Two aftershocks with moment magnitude of 5.0 and 4.9 occurred in the period of Fig. (a). An Mw5.1 aftershock occurred in the period of Fig. (b). Three aftershocks with moment magnitude of 5.3, 5.2 and 5. 5 occurred in the period of Fig. (c).
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Fig. 6. The 3D finite element models. The seismogenic faults highlighted with grid in (a) and (b) separately dip to south and north. The dip angles of two seismogenic faults are 600 and 450, (c) is the view of 3D model from the vertical direction. The lengths of the model along E–W and S–N are 100 km. The arrows denote the motion toward N60°E with the velocity of 14 mm/y and 20 mm/y.
Table 2 The elastic parameters of the 3D finite element models are applied in the forward simulation of the InSAR interferogram.
a b
Layer no.
Depth range (km)
Vpa (m/s)
Vsb (m/s)
Density (kg/m3)
Viscosity (Pa s)
1 2 3 4
0–3 3–22 22–42 42–64
2500.0 6000.0 6400.0 7100.0
1200.0 3500.0 3700.0 3900.0
2100.0 2700.0 2850.0 3050.0
– – 7.0e18 7.0e19
The velocities correspond to P wave. The velocities correspond to S wave.
structure’s study (Liu et al., 2006), our models are composed of four layers: 0–3 km, 3–22 km, 22–42 km and 42–64 km. The upper two layers are elastic, and the lower two layers are Maxwell viscoelastic. We separately fixed the viscosities of the lower two layers to 7.0 1018 Pa s and 7.0 1019 Pa s according to the previous study (Shao et al., 2008). The other elastic parameters are set by the Crust2.0 (Bassin et al., 2000). The details are listed in Table 2. The model lengths along east–west and along south–north are 100 km (see Fig. 6(c)). We meshed the top layer, the second layer
and the lower two layers by 600 m 600 m, 2 km 2 km and 5 km 5 km grid, respectively. The velocities toward N60°E on the south and north boundary are separately limited to 20 mm y 1 and 14 mm y 1 according to the previous observations (Wang et al., 2001; Cui et al., 2004). We applied quasi-static finite element algorithm (Williams et al., 2005; Aagaard et al., 2011) to model the InSAR observations. The co-seismic slip is derived from the teleseismic waveforms inversion. The post-seismic slip is determined by trial and error through visual assessment of the fit to InSAR
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Fig. 7. The modeled interferograms spanning the period from 28 November 2003 to 12 March 2004. (a) Corresponds to the seismogenic faults dipping to southwest, and (b) corresponds to the seismogenic faults dipping to northeast. The location of the black box is identical to that in Fig. 5. In (a), the interferometric fringes in the box are similar to the InSAR observations in Fig. 5.
observations. The co-seismic slip and the post-seismic slip are projected to the fault plane and are regarded as the known quantity. We modeled the co-seismic and the post-seismic deformation from the 16 April 2003 to 16 April 2004. Considering our InSAR observations only spanning three continuous periods of post-seismic deformation, we calculated the post-seismic deformation fields corresponding to three InSAR observations. Based on this, we computed the displacements in the light of sight of SAR from three components of the deformation fields, and then transferred them to the interferograms. We obtain the best seismogenic fault geometry using the following strategy: (a) to adjust the slip distribution and the dip of two faults, and model the InSAR interferograms; (b) to compare the modeled results with the observations, then re-adjust the slip distribution and the dip or obtain a preferred result. In this study, the east-fault is required to extend beneath the west-fault to fit the InSAR observations. The modeled InSAR interferograms spanning the period from 28 November 2003 to 12 March 2004 corresponding to the two models are shown in Fig. 7. Comparing the modeled results with InSAR observations in the black box, one can see that the modeled interferometric fringes of the southwest dipping faults (see Fig. 7a) is similar to the InSAR observations in Fig. 4, while the modeled result of the northeast dipping fault (see Fig. 7b) has no interferometric fringes in corresponding area. Therefore, we infer that the seismogenic faults are dipping to southwest. Because it is unknown of the exact rupture location and the length of the seismogenic faults and the post-seismic slip, the modeled results could not fully fit the observations. The optimal fit requires not only the exact dip, strike, length and depth information of the seismogenic faults, but also the slip distributions. Because the modeled InSAR interferograms are sensitive to the dip, the result of the seismogenic faults dipping to southwest is certain. 5. Discussion and conclusions The seismogenic faults of the 2003–2004 Delingha earthquakes are a part of the Dacaidan-Zongwulong fault system, which is an important active tectonic belt between the Qaidam basin and the Qilian Mountain. In this study, the teleseismic waveform data of the 2003 Mw6.3 Delingha earthquake and three InSAR observations
spanning all large aftershocks are analyzed. The co-seismic slip distribution from the seismic waveform inversion shows that the peak slip concentrates at 12 km in the depth direction and the slip decreased gradually from 6 km depth to surface and below 12 km depth. Through modeling the InSAR observations using the 3D finite element model, the seismogenic fault geometry of the 2003– 2004 Delingha earthquakes has been determined. There are two seismogenic faults: the east-fault and the west-fault. The east-fault extends beneath the west-fault. They dip to southwest, which is consistent with the result of Jiang et al. (2006). According to the focal mechanisms, the 2003 Mw6.3 Delingha earthquake occurred on the west-fault and the large aftershocks with similar magnitude occurred on the east-fault. The co-seismic slip distribution of the 2003 Mw6.3 Delingha earthquake is typical of ‘‘shallow slip deficit’’ proposed by Fialko et al. (2005). Two possible causes can explain the phenomena of ‘‘shallow slip deficit’’ (Fialko et al., 2005). One is that the interseismic strain accumulated in the uppermost 2–6 km of the seismogenic fault may be consumed by inelastic failures. Which leads to the low shear stress on the uppermost part of the seismogenic fault. The low shear stress may result in the ‘‘shallow slip deficit’’ phenomenon. The other is the velocity-strengthening friction behavior existing on the uppermost part of the seismogenic fault. For the latter, the strain energy may be accumulated on the uppermost part of the seismogenic fault, which will led to afterslip or aftershock. According to the earthquake catalog from China Earthquake data center, there is no aftershock occurred at the range of 0–5 km depth (see Fig. 5). However, the landslides had been observed in the range front near the epicenter region after the 2003 Mw6.3 Delingha earthquake. Therefore, the ‘‘shallow slip deficit’’ of the co-seismic slip distribution of the 2003 Mw6.3 Delingha earthquake may be caused by inelastic failures. While below the 12 km depth, the strain energy may be accumulated by the velocity-strengthening frition behavior (Wang et al., 2011) and may trigger aftershocks. Elliott et al. (2011) studied the 2008 and the 2009 Mw6.3 Qaidam earthquakes occurred in Daicaidan Country, located at the northwest of the Delingha earthquake’s epicenter. The seismogenic faults of those two earthquakes are nearly coplanar. The 2008 Qaidam earthquake ruptured the lower part of the seismogenic fault
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with relative small slip and the 2009 Qaidam earthquake broke the upper part with relative large slip. Elliott et al. (2011) proposed that the depth segmentation of the seismogenic continental crust allows a significant seismic hazard to remain after large earthquake. In this paper, our results show that the seismogenic fault of aftershocks extends beneath that of main shock. Which provides a new evidence for the viewpoint of Elliott et al. (2011). The earthquake can be triggered by the stress change induced by the preceding earthquake (King et al., 1994). The stress change can be realized by interactions of the tectonic blocks and viscoelastic relaxation of the lower crust and the upper mantle Freed, 2005; Sandy et al., 2005; Manga and Brodsky, 2006. Usually, viscoelastic relaxation of the lower crust and the upper mantle is very slow. Therefore, viscoelastic relaxation is considered as the cause of the aftershocks occurred in several or decades years after the main shock. In terms of 2003–2004 Delingha aftershocks, there is no direct evidence of the contribution of viscoelastic relaxation. The coseismic slip decreasing gradually beneath 12 km depth may be caused by the velocity-strengthening frition behavior. If it is true, the strain energy may be accumulated beneath 12 km depth of the west-fault plane and may trigger the deeper afterslip and aftershocks. According to the results from Elliott et al. (2011) and this paper, there exist many sub-faults in the Dacaidan-Zongwulong fault system, and the spatial location relationship between the sub-faults are extremely complex. We suggest that the complex location relationship between sub-faults might be an important factor to the large earthquakes occurrence with high frequency on almost the same seismogenic fault. In addition, the Dacaidan-Zongwulong fault system has been undergoing the pushing from the southern Tibetan plateau, the Qaidam basin and the Qilian Mountain. The strain energy is prone to be accumulated in this complex fault system, which may induce afterslip and aftershocks. Therefore, the Dacaidan-Zongwulong fault system will be a high seismic risk zone in the future. Acknowledgments This work was supported by the Natural Science Foundation of China (40804006) and the Fundamental Research Funds for the Central Universities. All Envisat SAR data sets were provided by Eurimage under CAT-1 research category (ERC-176). We thank Dr. Zheng Jianqiu generously provided the meterorological data. We also thank Dr. Wong Huihui and Dr. Zhang Miao for their help. Thoughtful reviews by an anonymous reviewer, Professor Zhao Dapeng and the Editor greatly improved the manuscript. References Aagaard, B., Kientz, S., Knepley, M., Somala, S., Strand, L., Williams, C., 2011. PyLith user manual (Version1.7.1). Computational Infrastructure of Geodynamics, 97– 151,. Abolghasem, A.M., Grafarend, E.W., 2003. Finite element analysis of quasi-static earthquake displacement fields observed by GPS. Journal of Geodesy 77, 529– 536.
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Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes
Constraints on the seismogenic faults of the 2003–2004 Delingha earthquakes by InSAR and modeling Xianjie Zha ⇑, Zhiyang Dai Laboratory of Seismology and Physics of Earth’s Interior, University of Science and Technology of China, Hefei 230026, Anhui Province, China
a r t i c l e
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Article history: Received 22 January 2013 Received in revised form 19 June 2013 Accepted 27 June 2013 Available online 9 July 2013 Keywords: Interferometric SAR Co-seismic slip Delingha earthquakes Numerical simulation
a b s t r a c t The 2003 Mw6.3 Delingha earthquake happened at the Dacaidan-Zongwulong fault system, which is an important active tectonic belt on the northeastern margin of Tibetan plateau. Until the end of 2004, 6 approximately Mw5.0 aftershocks occurred on almost the same seismogenic fault. The conventional viewpoint is that the seismic hazard near the continental seismogenic fault will be reduced in decades or centuries after rupture. Why did so many large earthquakes happen on almost the same seismogenic fault within a short temporal interval? The seismogenic fault information and the character of the co-seismic slip distribution are the keys to answer this problem. In this paper, the co-seismic slip distribution of the 2003 Mw6.3 Delingha earthquake was inverted using 18 teleseismic broadband P waveforms, 8 SH waveforms and 28 long period surface waveforms data. The peak slip concentrates at 12 km depth beneath the epicenter, and the slip decreases gradually from 6 km depth to surface and beneath 12 km depth. The shallow slip deficit of the co-seismic slip distribution may be caused by inelastic failures in the uppermost crust. Beneath 12 km depth, the strain energy may be accumulated by the velocity-strengthening frition behavior and may trigger aftershock. Three post-seismic InSAR interferograms spanning the most aftershocks were formed. Through analysis on the aftershock distribution and the focal mechanisms of Mw P 4.9 aftershocks, two 3D finite element models containing topography were built and used to simulate the InSAR observations. Comparing the modeled results with the InSAR observations, we obtained the seismic source of the 2003–2004 Delingha earthqakes. It consists of two southwest-dipping seismogenic faults, named the west-fault and the east-fault. The east-fault extends beneath the west-fault. The 2003 Mw6.3 Delingha earthquake occurred on the west-fault, while 6 large aftershocks occurred on the east-fault. This shows that the complex spatial location relationship between the seismogenic faults may be an important factor to the large earthquake occurrence with high frequency at almost the same locality. Due to special tectonic setting and complex structure, the Dacaidan-Zongwulong fault system will be a high seismic risk zone in the future. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction On 17 April 2003, an Mw6.3 earthquake hit Delingha region, Qinghai province, China. Until the end of 2004, more than 180 Mw P 2.0 aftershocks and 6 Mw P 4.9 aftershocks occurred in this region (see Table 1 and Fig. 1). According to the focal mechanisms given by GCMT and Sun et al. (2012), these earthquakes occurred on the west part of the Dacaidan-Zongwulong fault system, which is an important active tectonic belt between the Qaidam basin and the Qilian Mountain on the northeastern margin of the Tibetan Plateau (see Fig. 2). Moreover, the epicenters of Mw P 4.9 aftershocks are nearly located on the same seismogenic fault. Similarly, Elliott et al. (2011) studied the 2008 and the 2009 Mw6.3 Qaidam ⇑ Corresponding author. Tel.: +86 0551 63607203. E-mail address: [email protected] (X. Zha). 1367-9120/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2013.06.013
earthquakes and found that those earthquakes occurred on almost the same seismogenic fault in the different depths. Generally speaking, the seismic hazard of the seismogenic fault in the continental crust will be reduced in serval decades or centuries after the rupture releasing the strain energy. Why can so many earthquakes with similar magnitude happen on almost the same seismogenic fault within a short time interval?. The seismogenic fault geometry and the character of the coseismic slip distribution are the keys to answer the above problem. In addition, the seismogenic fault information is the base to study seismic source, seismic risk, regional geodynamics, and so on. Due to the sharp topography in the Delingha region, it is very difficult to study the seismogenic faults by the field geological investigation. According to the previous results, Jiang et al. (2006) inferred that the dip of the seismogenic faults is toward southwest (see Fig. 3 in Jiang’s paper). However, Sun et al. (2012) studied a part
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Table 1 The focal mechanism solutions of the 2003 Mw6.3 Delingha earthquake and its aftershocks. The data is derived from GCMT.
a b
Time
Longt. (°)
Lati. (°)
Dep. (km)
Magn. (Mw)
Fault Strike
Plane Dip
2003/4/17 2004/2/24 2004/3/02 2004/3/16 2004/5/04 2004/5/04 2004/5/10
96.45 96.61 96.52 96.56 96.65 96.62 96.58
37.53 37.66 37.61 37.60 37.60 37.61 37.55
16.0 23.0 21.1 13.0 18.6 21.6 12.0
6.3 5.0 4.9 5.1 5.3 5.2 5.5
116 141 100 127 145 140 116
61 87 41 42 83 87 59
Solutiona Rake 91 179 93 116 175 178 103
Typeb Thrust-slip Strike-slip Thrust-slip Thrust & strike-slip Strike-slip Strike-slip Thrust & strike-slip
The fault plane solution is inferred from the GCMT focal mechanisms and the distribution of aftershocks with Mw P 2.0. The fault type is derived from the focal mechanism.
of aftershocks of the 2003 Mw6.3 Delingha earthquake and inferred that the seismogenic faults are dipping to northeast according to the aftershock distribution. Up to now, the seismogenic fault geometry of the 2003–2004 Delingha earthquakes is still unkown. In addition, Elliott et al. (2011) proposed that depth segmentation of the seismogenic continental crust allows a significant seismic hazard to remain after large earthquake. To look for the cause of the frequent occurrence of large earthquakes on almost the same seismogenic fault, this paper used the InSAR observations, the teleseismic waveform data and numerical simulation to constrain the seismogenic fault and analyzed the seismic risk of the Dacaidan-Zongwulong fault system.
Fig. 1. The 2003–2004 Delingha earthquake series map. The data is derived from the Network of Earthquake Information of China. The local magnitude of the main shock is 6.7, corresponding to moment magnitude 6.3. Until the end of 2004, there are more than 180 aftershocks (6 aftershocks with moment magnitude P4.9) occurred in the source region.
Fig. 2. The tectonic setting map. The Dacaidan-Zongwulong fault is an important active tectonic belt between the Qaidam basin and the Qilan mountain, on the northeastern margin of Tibetan Plateau. Open stars denoting the 2003–2004 earthquakes occurred on the west part of the Dacaidan-Zongwulong fault system. The topography of this region is very sharp.
2. Co-seismic slip inversion Before the 2003 Mw6.3 Delingha earthquake, no SAR data covering the source region is available. Because the moment magnitude is up to 6.3, the main shock could be recorded by the global distributed seismic stations, whose epicenter distances are from 30° to 90°. The co-seismic slip distribution can be inverted using teleseismic broadband waveform data. To invert the co-seismic slip of the 2003 Mw6.3 Delingha earthquake, we collected the focal mechanisms from GCMT and the GSN broadband waveforms data from the IRIS seismic network. Based on data quality and azimuthally distribution, we analysed 18 teleseismic broadband P waveforms, 8 SH waveforms and 28 long period surface waveforms data. Two possible fault planes are separately chosen to invert the slip distribution. One fault plane’s strike, dip and rake are 116°, 61° and 91° separately. The other’s are 294°, 29° and 88° separately. The epicenter is located at (96.45°E, 37.53°N). The 1D crustal model is from the Crust2.0 (Bassin et al., 2000). We utilized the finite fault inversion algorithm to invert the co-seismic slip distribution of the 2003 Mw6.3 Delingha earthquake. For the details of the inversion technique, please refer to the paper of Ji et al. (2002). Through comparing the waveform fits, we found the first fault plane (strike = 116°, dip = 61°, rake = 91°) fitting the data is better. In Fig. 3, (a) is the distribution of the seismic stations, (b) is the source time function of the 2003 Mw6.3 Delingha earthquake, (c) and (d) are the observed and the modeled body waveforms and surface waveforms, respectively. The co-seismic slip distribution is shown in (e). In figure (e), one can see that the slip distribution extends from 2 to 20 km in depth direction. The peak slip is up to 40 cm, which is concentrated at 12 km depth beneath the epicenter. The slip distribution is asymmetrical, and the slip magnitude on the east side of the fault plane is larger than that of the west side. From 6 km depth to surface and below 12 km depth, the slip decreases gradually. The rupture length on the Earth’s surface is about 20 km, but that in the depth of 12 km is more than 30 km.
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Fig. 3. Co-seismic slip distribution and observed (black) and modeled (red) teleseismic waveforms. (a) Is the station distribution. The star denotes the seismic source, the triangles denote the seismic station, (b) is the source time function of the event, (c and d) are separately indicating the body waveform and surface waveform fits, (e) is the co-seismic slip distribution of the 2003 Mw6.3 Delingha earthquake. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Post-seismic InSAR observations After the main shock, the Advanced SAR sensor of the Envisat satellite acquired many SAR data sets covering the source region
of the 2003 Mw6.3 Delingha earthquake. We collected 7 scenes SAR data sets from European Space Agency. In addition, we collected the meteorological data of the Delingha region. The meteorological conditions of the Delingha during 2003–2004 are very
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favorable to map the deforation by InSAR. In terms of less vegetation in this region, the deformation interferogram derived from the collected SAR data sets should be with good coherence. However, we processed more than 8 InSAR image pairs with small perpendicular and temporal baseline, and found that the most pairs could not produce the ideal interferometric fringes. After the events, Sun et al. (2003) reported that landslides occurred in the epicenter region. This is a possible explanation to the low coherence of SAR image pairs. At the southeast corner of the coverage of the SAR image, there is a flat area. The correponding interferometric fringes in three InSAR maps is very clear (see Fig. 4). Those are the only observations of the post-seismic process in the near field. Three interferograms span the period from 28 November 2003 to 25 June 2004. Following the 2003 Mw6.3 Delingha earthquake, a series of aftershocks happened in the source region. Two aftershocks with moment magnitude of 5.0 and 4.9 occurred in the period of Fig. 4(a). An Mw5.1 aftershock occurred in the period of Fig. 4(b). Three aftershocks with moment magnitude of 5.3, 5.2 and 5.5 occurred in the period of Fig. 4(c). Therefore, the deformations induced by these aftershocks should be recorded by three post-seismic interferograms. 4. The seismogenic fault The seismogenic fault geometry is an important basic information to study the process of seismic source (Wang et al., 2012; Shan et al., 2011). According to the aftershock location and the focal mechanisms of the Mw P 4.9 earthquakes from Sun et al. (2012) and GCMT, we inferred that there may exist two seismogenic faults, whose strikes are about 116° and 140° separately (see Table 1 and Fig. 5). In this paper, the seismogenic faults are named the west-fault and the east-fault. According to the focal mechanism of the 2003 Mw6.3 Delingha earthquake, the west-fault is the seismogenic fault of the main shock. We assumed that its length along strike is 40 km and its dip angle is 60°. From the coseismic slip distribution, we inferred that the depth of west-fault is about 20 km. However, the parameters of the east-fault are uncertain. We assurmed that its length along strike is about 56 km and its dip angle is 45°. Because the depths of some aftershocks near the east-fault are larger than 20 km, the east-fault may cut through the upper crust and its depth approximates to 22 km. The finite element method is widely used to model the deformation field induced by large earthquakes (Hu et al., 2004; Wang
Fig. 5. Aftershock locations and focal mechanisms of the 2003–2004 Delingha earthquakes. The open cycles denote the aftershock distribution in the Earth’s surface, and the crosses denote the aftershock distribution in the depth direction. The focal mechanism solutions are determined by GCMT. Two red lines denote the strike of the seismogenic fault inferred from focal mechanisms and aftershock distribution. The dashed box denotes the coverage of the SAR data used in this paper. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
et al., 2007; Abolghasem and Grafarend, 2003). To further constrain the dips of the seismogenic faults of the 2003–2004 Delingha earthquakes, we built two 3D finite element models containing topography with different dips to model the InSAR obervations. In Fig. 6, (a) and (b) are the view of the 3D finite element models. The seismogenic faults’ dips in (a) and (b) are separately toward southwest and northeast. According to the previous crustal
Fig. 4. The post-seismic interferograms of the 2003 Mw6.3 Delingha earthquake. Two aftershocks with moment magnitude of 5.0 and 4.9 occurred in the period of Fig. (a). An Mw5.1 aftershock occurred in the period of Fig. (b). Three aftershocks with moment magnitude of 5.3, 5.2 and 5. 5 occurred in the period of Fig. (c).
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Fig. 6. The 3D finite element models. The seismogenic faults highlighted with grid in (a) and (b) separately dip to south and north. The dip angles of two seismogenic faults are 600 and 450, (c) is the view of 3D model from the vertical direction. The lengths of the model along E–W and S–N are 100 km. The arrows denote the motion toward N60°E with the velocity of 14 mm/y and 20 mm/y.
Table 2 The elastic parameters of the 3D finite element models are applied in the forward simulation of the InSAR interferogram.
a b
Layer no.
Depth range (km)
Vpa (m/s)
Vsb (m/s)
Density (kg/m3)
Viscosity (Pa s)
1 2 3 4
0–3 3–22 22–42 42–64
2500.0 6000.0 6400.0 7100.0
1200.0 3500.0 3700.0 3900.0
2100.0 2700.0 2850.0 3050.0
– – 7.0e18 7.0e19
The velocities correspond to P wave. The velocities correspond to S wave.
structure’s study (Liu et al., 2006), our models are composed of four layers: 0–3 km, 3–22 km, 22–42 km and 42–64 km. The upper two layers are elastic, and the lower two layers are Maxwell viscoelastic. We separately fixed the viscosities of the lower two layers to 7.0 1018 Pa s and 7.0 1019 Pa s according to the previous study (Shao et al., 2008). The other elastic parameters are set by the Crust2.0 (Bassin et al., 2000). The details are listed in Table 2. The model lengths along east–west and along south–north are 100 km (see Fig. 6(c)). We meshed the top layer, the second layer
and the lower two layers by 600 m 600 m, 2 km 2 km and 5 km 5 km grid, respectively. The velocities toward N60°E on the south and north boundary are separately limited to 20 mm y 1 and 14 mm y 1 according to the previous observations (Wang et al., 2001; Cui et al., 2004). We applied quasi-static finite element algorithm (Williams et al., 2005; Aagaard et al., 2011) to model the InSAR observations. The co-seismic slip is derived from the teleseismic waveforms inversion. The post-seismic slip is determined by trial and error through visual assessment of the fit to InSAR
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Fig. 7. The modeled interferograms spanning the period from 28 November 2003 to 12 March 2004. (a) Corresponds to the seismogenic faults dipping to southwest, and (b) corresponds to the seismogenic faults dipping to northeast. The location of the black box is identical to that in Fig. 5. In (a), the interferometric fringes in the box are similar to the InSAR observations in Fig. 5.
observations. The co-seismic slip and the post-seismic slip are projected to the fault plane and are regarded as the known quantity. We modeled the co-seismic and the post-seismic deformation from the 16 April 2003 to 16 April 2004. Considering our InSAR observations only spanning three continuous periods of post-seismic deformation, we calculated the post-seismic deformation fields corresponding to three InSAR observations. Based on this, we computed the displacements in the light of sight of SAR from three components of the deformation fields, and then transferred them to the interferograms. We obtain the best seismogenic fault geometry using the following strategy: (a) to adjust the slip distribution and the dip of two faults, and model the InSAR interferograms; (b) to compare the modeled results with the observations, then re-adjust the slip distribution and the dip or obtain a preferred result. In this study, the east-fault is required to extend beneath the west-fault to fit the InSAR observations. The modeled InSAR interferograms spanning the period from 28 November 2003 to 12 March 2004 corresponding to the two models are shown in Fig. 7. Comparing the modeled results with InSAR observations in the black box, one can see that the modeled interferometric fringes of the southwest dipping faults (see Fig. 7a) is similar to the InSAR observations in Fig. 4, while the modeled result of the northeast dipping fault (see Fig. 7b) has no interferometric fringes in corresponding area. Therefore, we infer that the seismogenic faults are dipping to southwest. Because it is unknown of the exact rupture location and the length of the seismogenic faults and the post-seismic slip, the modeled results could not fully fit the observations. The optimal fit requires not only the exact dip, strike, length and depth information of the seismogenic faults, but also the slip distributions. Because the modeled InSAR interferograms are sensitive to the dip, the result of the seismogenic faults dipping to southwest is certain. 5. Discussion and conclusions The seismogenic faults of the 2003–2004 Delingha earthquakes are a part of the Dacaidan-Zongwulong fault system, which is an important active tectonic belt between the Qaidam basin and the Qilian Mountain. In this study, the teleseismic waveform data of the 2003 Mw6.3 Delingha earthquake and three InSAR observations
spanning all large aftershocks are analyzed. The co-seismic slip distribution from the seismic waveform inversion shows that the peak slip concentrates at 12 km in the depth direction and the slip decreased gradually from 6 km depth to surface and below 12 km depth. Through modeling the InSAR observations using the 3D finite element model, the seismogenic fault geometry of the 2003– 2004 Delingha earthquakes has been determined. There are two seismogenic faults: the east-fault and the west-fault. The east-fault extends beneath the west-fault. They dip to southwest, which is consistent with the result of Jiang et al. (2006). According to the focal mechanisms, the 2003 Mw6.3 Delingha earthquake occurred on the west-fault and the large aftershocks with similar magnitude occurred on the east-fault. The co-seismic slip distribution of the 2003 Mw6.3 Delingha earthquake is typical of ‘‘shallow slip deficit’’ proposed by Fialko et al. (2005). Two possible causes can explain the phenomena of ‘‘shallow slip deficit’’ (Fialko et al., 2005). One is that the interseismic strain accumulated in the uppermost 2–6 km of the seismogenic fault may be consumed by inelastic failures. Which leads to the low shear stress on the uppermost part of the seismogenic fault. The low shear stress may result in the ‘‘shallow slip deficit’’ phenomenon. The other is the velocity-strengthening friction behavior existing on the uppermost part of the seismogenic fault. For the latter, the strain energy may be accumulated on the uppermost part of the seismogenic fault, which will led to afterslip or aftershock. According to the earthquake catalog from China Earthquake data center, there is no aftershock occurred at the range of 0–5 km depth (see Fig. 5). However, the landslides had been observed in the range front near the epicenter region after the 2003 Mw6.3 Delingha earthquake. Therefore, the ‘‘shallow slip deficit’’ of the co-seismic slip distribution of the 2003 Mw6.3 Delingha earthquake may be caused by inelastic failures. While below the 12 km depth, the strain energy may be accumulated by the velocity-strengthening frition behavior (Wang et al., 2011) and may trigger aftershocks. Elliott et al. (2011) studied the 2008 and the 2009 Mw6.3 Qaidam earthquakes occurred in Daicaidan Country, located at the northwest of the Delingha earthquake’s epicenter. The seismogenic faults of those two earthquakes are nearly coplanar. The 2008 Qaidam earthquake ruptured the lower part of the seismogenic fault
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with relative small slip and the 2009 Qaidam earthquake broke the upper part with relative large slip. Elliott et al. (2011) proposed that the depth segmentation of the seismogenic continental crust allows a significant seismic hazard to remain after large earthquake. In this paper, our results show that the seismogenic fault of aftershocks extends beneath that of main shock. Which provides a new evidence for the viewpoint of Elliott et al. (2011). The earthquake can be triggered by the stress change induced by the preceding earthquake (King et al., 1994). The stress change can be realized by interactions of the tectonic blocks and viscoelastic relaxation of the lower crust and the upper mantle Freed, 2005; Sandy et al., 2005; Manga and Brodsky, 2006. Usually, viscoelastic relaxation of the lower crust and the upper mantle is very slow. Therefore, viscoelastic relaxation is considered as the cause of the aftershocks occurred in several or decades years after the main shock. In terms of 2003–2004 Delingha aftershocks, there is no direct evidence of the contribution of viscoelastic relaxation. The coseismic slip decreasing gradually beneath 12 km depth may be caused by the velocity-strengthening frition behavior. If it is true, the strain energy may be accumulated beneath 12 km depth of the west-fault plane and may trigger the deeper afterslip and aftershocks. According to the results from Elliott et al. (2011) and this paper, there exist many sub-faults in the Dacaidan-Zongwulong fault system, and the spatial location relationship between the sub-faults are extremely complex. We suggest that the complex location relationship between sub-faults might be an important factor to the large earthquakes occurrence with high frequency on almost the same seismogenic fault. In addition, the Dacaidan-Zongwulong fault system has been undergoing the pushing from the southern Tibetan plateau, the Qaidam basin and the Qilian Mountain. The strain energy is prone to be accumulated in this complex fault system, which may induce afterslip and aftershocks. Therefore, the Dacaidan-Zongwulong fault system will be a high seismic risk zone in the future. Acknowledgments This work was supported by the Natural Science Foundation of China (40804006) and the Fundamental Research Funds for the Central Universities. All Envisat SAR data sets were provided by Eurimage under CAT-1 research category (ERC-176). We thank Dr. Zheng Jianqiu generously provided the meterorological data. We also thank Dr. Wong Huihui and Dr. Zhang Miao for their help. Thoughtful reviews by an anonymous reviewer, Professor Zhao Dapeng and the Editor greatly improved the manuscript. References Aagaard, B., Kientz, S., Knepley, M., Somala, S., Strand, L., Williams, C., 2011. PyLith user manual (Version1.7.1). Computational Infrastructure of Geodynamics, 97– 151,
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